A gestation wheel, also referred to as “a pregnancy wheel,” is a commonly used calculation device used by physicians, mid-wives, pregnant women, veterinarians, and the general public to determine the course of a pregnancy, and in particular the estimated date of delivery of a pregnancy. The gestation wheel calculating device has been in general use for several decades, and is invaluable in identifying the calendar dates for different landmarks and phases of pregnancy. The typical embodiment of this device is a circular piece of paper, plastic, or metal between 4 inches and 10 inches in diameter; a 365-day radial calendar printed along the outer circumference on one side; and a separate slightly smaller diameter circular component made of the same material which freely rotates on top of the calendar component by means of a center pin. The smaller rotating circular piece upon which various landmarks of pregnancy are printed is aligned with the calendar component in order to calculate the calendar dates for any pregnancy. The pregnancy landmarks typically consist of a marked position for the last menstrual period, a marked position for the date of delivery for a normal pregnancy, and markers placed seven “days” apart to display and label the number of weeks between the last menstrual period and the delivery date. Because a typical human pregnancy extends 40 weeks from last menstrual period to delivery date, a total of 40 weeks are usually labeled on the rotating wheel. The device is operated by rotating the last menstrual period marker to align with its calendar date, then reading the calendar date aligned with the delivery date marker to identify the estimated “due date” of that particular pregnancy. Any calendar date between these two days will automatically align with the associated “gestational age.” Other pregnancy landmarks that correlate with gestational age may also be printed on the marker wheel. These may include segments labeling the first, second, and third trimester; segments that identify the best time to perform tests during pregnancy, such as amniocentesis or diabetes tests; or markers identifying typical fetal weight or fetal ultrasound measurements corresponding to various gestational ages.
Human pregnancy is measured in terms of gestational age and conceptual age. Gestational age is the most commonly used measurement for ongoing pregnancy, and is a relic of the middle ages before egg and sperm physiology was discovered. Essentially it measures the duration of pregnancy from the first day of a woman's most recent menstrual period. The average length of a full-term pregnancy in terms of gestational age is 40 weeks (or 280 days). After the discovery of egg and sperm fertilization, conceptual age was used to more accurately determine the actual duration of pregnancy. Because conception occurs on average 14 days after the onset of the menstrual period, the average duration of pregnancy to full-term delivery is 38 weeks (or 266 days). Despite the increased accuracy of conceptual age, the great majority of gestation wheel calculating devices still use gestational age as the primary unit of measurement for pregnancy duration.
The average duration of a menstrual cycle is 28 days, measured from the first day of one menstrual period to the first day of the following menstrual period. Ovulation typically occurs 14 days after the first day of the period (cycle day 14), and because the lifespan of the egg is only 28 hours, fertilization and conception typically occur less than 1 day after ovulation. Although 28 days is the average menstrual cycle length, the normal range varies from 24 days to 38 days, with this range encompassing 99.5% of all regular menstrual cycles. Approximately 65% of menstrual cycle lengths fall within the range of 26 to 32 days. The variable portion of menstrual cycles is in the period before ovulation. The subsequent period after ovulation is virtually always 14 days long. For a 28-day menstrual cycle, 14 days lie before ovulation and 14 days lie after ovulation. For a 34-day menstrual cycle, 20 days lie before the ovulation and 14 days lie after ovulation.
Several physiologic landmarks occur around ovulation and extend through the first trimester of pregnancy. Ovulation is triggered by a surge in the luteinizing hormone (LH) which occurs 24 to 40 hours before ovulation. This LH surge can be detected by a commercial urine test kit which contains a pad of filter paper sensitive to the LH hormone. The pad of filter paper will become brightly colored on the day immediately preceding ovulation, and is typically used by couples to time sexual intercourse in order to attempt pregnancy. The average lifespan for sperm is 2 to 3 days, although on relatively rare occasions, a small number of sperm can survive as long as 5½ days and still be capable of fertilizing an egg. Timed intercourse to achieve pregnancy is best done on the day of ovulation, and is also acceptable 2 to 3 days before ovulation. In unusual cases timed intercourse 5½ days before ovulation can result in pregnancy. Because the lifespan of the oocyte is 28 hours, timed intercourse more than 24 hours after ovulation is too late to achieve pregnancy, taking into account that it takes over 3 hours for sperm to reach the fertilization site in the fallopian tube.
Once the egg fuses with the sperm, the combined cell is called a zygote. The first cell division occurs 24 hours later, and subsequent cell divisions occur approximately 10 hours apart thereafter. Therefore two days after ovulation the embryo is at the four cell stage, three days after fertilization the embryo is at the eight to twelve cell stage, four days after ovulation the embryo is at the 32 to 128 cell stage and is known as a morula, and five days after fertilization the embryo is known as a blastocyst when it develops a central fluid-filled cavity. The next day (six days after fertilization), the embryo hatches out of a clear thin zona shell, then floats freely in the uterine cavity. The following day the embryo implants into the inner wall of the uterus and acquires a blood supply for further development. Over the next few days the outer placenta circumference enlarges and a fluid-filled amniotic cavity forms, and the embryo secretes enough β-hCG hormone into the bloodstream that its presence can be detected by maternal blood pregnancy test sensitive to this hormone. The blood pregnancy test generally turns positive nine to twelve days after fertilization, and the gestational sac becomes large enough to be visualized on an ultrasound test as a gestational sac approximately 11 to 21 days after fertilization; Within five to nine days, a fetal pole becomes visible on ultrasound inside the gestational sac, with this fetal pole initially growing approximately 1 mm per day in length (the crown-rump length or CRL). Two to five days after the fetal pole becomes visible, the fetal heart develops to the point that a steady beating heart motion is detected on ultrasound, initially around 60 to 90 beats per minutes, then increasing to 110 to 150 beats per minute (fetal heart motion or FHM). Over the next two weeks, the gestational sac and the fetus become larger with more details visible on ultrasound, and a thin membrane surrounding the fetus known as the amnion sac becomes visible.
The gestational age ranges for development of specific organ systems is well documented. The beginning, mid-range, and ending developmental phase for organs such as the eye, genitalia, arms, legs, and heart are well established and this period is known as organogenesis. If environmental or other insults occur in the developing tissues during this period, birth defects in a particular organ system may result. After the organogenesis phase, the fetus simply grows larger and larger until delivery, so environmental insults occurring during the growth phase may cause retarded or accelerated growth and result in low birth weight or high birth weight infants, but insults during the growth phase do not cause birth defects.
Various organs and fetal structures can be measured by gestational ultrasound during pregnancy, and standard tables of these measurements have been developed, with growth curves calculated for each gestational age, complete with normal ranges and standard deviations. Examples of gestational ultrasound measurements include transverse skull measurement (biparietal diameter or BPD), length of the femur bone (FL), circumference of the abdomen and head (AC and HC), diameter of the cerebellum, and distance between eyeballs. These measurements can then be used to determine secondary factors such as estimated fetal weight or head circumference to abdominal circumference ratio by applying the appropriate algorithm. Each of these secondary calculation factors has its own growth curve and normal range standards when compared to gestational age.
A large number of pregnancy medical tests must be performed within a narrow range of gestational age due to physiologic restrictions. For instance the chorionic villus sampling test (CVS) must be done between 10 and 12 week gestation, and the amniocentesis test is best done between 14 and 16 weeks gestation. Gestation calculating wheels are often used to determine the calendar date of these tests for individual pregnancies. A very important task of medical personnel during pregnancy is the accurate determination of the most likely day of full-term delivery (due date or estimated date of confinement “EDC”). Three weeks before and two weeks after the due date is considered term, and is the safe time to delivery the baby. Delivery before term may result in prematurity with its associated problems. Deliveries after term are considered post due or post term, and these deliveries are also associated with a very high complication rate.
Most gestation calculating wheels contain only three types of information marked on the rotating plate—1) the first day of last menstrual period, 2) the due date, and 3) the weeks of gestational age in between. Other information is occasionally included such as scheduling periods for various pregnancy tests like CVS and amniocentesis, between the appropriate gestational age week markers.
Prior art wheels include those disclosed in: U.S. Pat. Nos. 4,737,619; 2,727,686; 3,278,118; 2,418,207; 4,350,878; 3,486,691; 3,771,716; and 2,808,206; US Publication No. 2003/0024974; PCT publication nos. WO 97/33214 and WO 01/36212; and web sites http://www.cdphe.state.co.us/ps/bestpractices/topicsubpages/inadequateweightgain.html; and www.pregnancyplanningguide.com/about_pregnancy.cfm.
Gestational calculators often contain additional space which is not involved with the calculation of gestational markers. The additional space is usually located near the center of the wheel inside the gestational markers, on the entire back side of the base plate, on the ¼ circumference of the gestational plate between the due date and the last menstrual period markers. Some of this space is available because human gestations are approximately nine months in length, and the remaining three months of the year have no associated markers.
The physical structure of most gestation wheels consists of a circular (or less often rectangular) “base plate” upon which a 365-day calendar is printed along the outer circumference. The months of the year are printed sequentially around the outermost circumference, dividing the base plate into 12 monthly segments, and the days of each month are printed within as short lines along a circumference immediately inside the month labels. Typically every five or ten days is numerically labeled in order to locate a specific calendar date. For instance, within the segment labeled January, every 5th mark is labeled with a numeral “5, 10, 15, 20, 30.” January 12th would be the second marker after the one labeled “10.” The center point of the calendar base plate contains a pin which connects the base plate to a second circular plate which freely rotates concentrically over the base plate. This is the gestational plate, and printed radially upon the outer circumference of this plate are markers for the typical landmarks of pregnancy. This nearly always includes a marker for the first day of the “last menstrual period,” a marker for the “due date,” and markers located seven days apart and occupying approximately nine months duration between the menstrual period and “due date” marker. These are each labeled with a number representing the gestational age in units of weeks. For instance the marker labeled 22 would mark the point on the circumference associated with the 22nd week after the last menstrual period. By rotating the gestational plate along the base plate, the last menstrual period marker can be directly aligned with the calendar date for a particular pregnancy. For example, if the patient's last menstrual period lasted from March 4th to March 8th, the last menstrual period marker would be aligned directly on top of the March 4th marker on the base plate. Once this is done, the calendar date of all other gestational markers, including the due date, can be read on the calculator when the two plates are held fixed in this position. It is common that the gestational age weekly marks extend to 42 or 44 weeks gestation in order to accommodate post-due pregnancies. Some of the more modern gestational calculators will also contain markers for LH surge, ovulation, fertilization, and even a few phases of embryo or fetal development.